A well-known solution in the case of knee joints which have been damaged by wear or by other conditions such as, for example, tumors, and which no longer allow the patient concerned to move in a normal and, in particular, pain-free manner, is to replace the joint surfaces of the bones concerned, in other words, on the distal end of the femur and on the dorsal tibia side, with suitable implants. For this purpose, various renowned commercial suppliers provide suitable implants or implant families, which are now routinely inserted into affected patients.
Different implant sizes must be used due to differing anatomical proportions in the patients concerned. For example, patients have differently sized bone sections or bone ends at the distal femur, but also at the dorsal tibia. For this purpose, the suppliers offer, usually in the respective implant series, suitable implants (femoral implants for replacement of the distal femoral joint surfaces, tibial implants for replacement of the dorsal tibia joint surfaces) in different sizes. Sets with between 5 and 10 different implant sizes are common.
Although preoperative diagnostics, in particular using X-ray images or CT scans, provides the attending physician and surgeon with initial indications not only regarding the finding of wear or of pathological damage of the knee joint, but also regarding the proportions of the natural joint mechanism, it is nevertheless not possible to undertake precise sizing of the respective implant (femoral implant, tibial implant) to be used solely on the basis of the preoperative diagnostics, and the surgeon still has to make an appropriate choice during the operation. The surgeon additionally often obtains a definitive diagnostic picture only during the operation and, in addition to determining the selectable size of the respective implant part, in particular of the femoral implant, he also determines the precise alignment for the attachment of this implant part to the remaining bone (in particular to the section of the distal femur remaining after the required resection cutting). In many cases, an arrangement of the implant aligned along the axis extending from medial to lateral on the plateau formed by the distal femur cut is not selected, instead, an alignment which is inclined relative to this axis by an external implant angle is set, said inclination typically being able to be in a range between approx. 0 and 7°, typically inclined from medial to lateral.
In a typical procedure for carrying out the preparatory resections for the femoral implant which is to be selected with respect to the suitable size, the distal condyle cut is firstly realized once the patient's knee joint has been exposed with the knee in a bent position, in other words, in flexion (the bent position typically being approximately 90°), in which procedure a first planar cut surface is created at the point at which the distal condyles, in other words joint surfaces, of the femur are arranged. Subsequently, the determination of the implant size which is suitable for the patient is undertaken typically using a jig by means of measurement of the anatomical dimensions from anterior to posterior. The jig has a main body, a probe part displaceable relative to the main body along a measuring direction, wherein the probe part has a probe arm with a probe tip for contacting on an anterior reference point (RP) of the distal end of the patient's femur, two contact pieces for contacting on and referencing on respectively the medial and the lateral posterior condyle of the distal end of the patient's femur and at least one scale and an associated indicator for indicating a location of the probe part in the measuring direction relative to at least one of the contact pieces for the specification of a suitable implant size. For this purpose, the jig with the contact pieces is placed under the posterior condyles, which lie on the tibial plateau in the flexion position, and by means of the probe arm, or more specifically by means of its probe tip, an anterior reference point on the femur is contacted which the surgeon considers to be suitable, typically the uppermost anterior point which is situated dorsal relative to the anterior condyle end portions. The probe part is correspondingly displaced in the measuring direction, and it is then possible to read an implant size which is suitable for the patient from the scale by means of the location of the indicator relative to this scale.
Examples of such jigs are described together with the corresponding procedure for their use for example in WO 94/00056 A1, WO 2005/046432 A2, U.S. Pat. Nos. 5,624,444, 5,810,831, US 2004/0220583 A1 and US 2009/0143783 A1.
In addition, jigs of the type that have a main body, a probe part displaceable relative to the main body along a measuring direction, wherein the probe part has a probe arm with a probe tip for contacting on an anterior reference point (RP) of the distal end of the patient's femur, two contact pieces for contacting on and referencing on respectively the medial and the lateral posterior condyle of the distal end of the patient's femur and at least one scale and an associated indicator for indicating a location of the probe part in the measuring direction relative to at least one of the contact pieces for the specification of a suitable implant size are known, in which devices for the specification of an external rotation angle of the implant are additionally provided on the main body, said devices having pinholes through which, after setting of the desired rotation, corresponding pins can be driven into the first contact surface of the implant created by the distal condyle cut, for the purpose of further orientation and alignment of the implant in its angular position relative to a line extending from medial to lateral by means of the orientation of the cutting jig, correspondingly carried out with the help of these pins, for the additional resection cuts to be realized for the shaping of the femur. One example of such a jig is described in DE 60 2004 011 420 T2, another example is the jig presented by the US supplier Zimmer. Inc. in their brochure “Zimmer® NexGen® CR-Flex and LPS-Flex Knees Surgical Technique with Posterior Referencing Instrumentation”, which comprises on the main body, in addition to the probe part, another rotating body which can be pivoted about a pivot pin, by means of which the corresponding external angular setting can be specified and in which the above-mentioned pinholes are formed.
A common factor in all of the above-mentioned known jigs for determining the patient-adapted implant size of the femoral implant of a knee endoprosthesis is that the contact pieces for the contacting on and for the referencing on the medial and the lateral posterior condyle respectively are firmly connected to the main body so that, relative thereto, only the probe part can be moved in the measuring direction for the determination of the size.
However, in particular when specification of an external angular setting is to be realized together with the determination of size, this design can lead to measuring errors and consequently to problems when selecting the suitable implant size.